Myogenic stem cell cultures derived from the regenerating tail of the lizard Anolis carolinensis have never been observed to enter Hayflick's Phase III of culture (Simpson and Cox, 1972; Simpson and Bayne, 1978). To date five continuous culture lineages have been established. One clonal myogenic lineage has been carried in continuous culture for five years (1,100 cell generations). Lizards providing cells for the establishment of this culture lineage were at least 2 years old. The maximum recorded life spin of Anolis carolinensis in captivity is 5 years (Burrage, 1964). This lineage has therefore been carried in continuous culture for a period exceeding the probable life expectancy of the species. Lizard myogenic culture lineages can be maintained at between 80-90 percent diploid, and by a variety of criteria are not transformed. The lizard myogenic system is a striking exception to the Hayflick (1965) hypothesis of limited in vitro proliferation potential (i.e. cellular aging in vitro). The proposed project has three main objectives, namely to provide answers to the following questions: a) Is the unlimited proliferation potential of lizard myogenic stem cells unique to muscle or is it a property of lizard cells in general? b) Is the observed unlimited proliferation potential of lizard myogenic stem cells a consequence of our culture regime and our ability to select for 'normal' cells? c) Is the unlimited proliferation potential of myogenic stem cells derived from lizard tail regenerates tied to their ability to participate in epimorphic regeneration?